In small communications apparatuses, such as mobile telephones, acoustic wave devices are commonly used as high-frequency filters having passing frequency range from tens of megahertz to several gigahertz.
FIG. 3A is a top view of a piezoelectric substrate 2 of a conventional acoustic wave device 1. FIG. 3B is a cross sectional view of the acoustic wave device 1 along line 3B-3B shown in FIG. 3A. The acoustic wave device 1 includes the piezoelectric substrate 2 made of a single crystal piezoelectric substance, such as lithium tantalate or lithium niobate, and comb-shaped electrodes 3 disposed on a surface of the piezoelectric substrate 2. The acoustic wave device 1 forms a filter circuit with comb-shaped electrodes 3 generating acoustic waves.
In the acoustic wave device 1, high-frequency currents flowing in the comb-shaped electrodes 3 affect each other particularly at a point where one wiring connected to the comb-shaped electrode 3 is close to another wiring. This influence generates a parasitic factor, such as a parasitic capacitance or inductance, hence causing insertion loss or decay to decline the high-frequency filtering characteristics of the acoustic wave device 1. Since a single crystal piezoelectric substance applicable to the piezoelectric substrate 2 generally is a highly dielectric material having a dielectric constant typically higher than 20, wirings disposed on the piezoelectric substrate 2 likely generate such parasitic factor. For suppressing the parasitic factors, an insulator 6 is disposed between wirings 4 and 5 which cross one over the other, another insulator 8 is disposed between the piezoelectric substrate 2 and a wiring 7, or a further insulator 10 is disposed on a wiring 9. The insulators 6, 8, and 10 are made of a resin material which can easily form a thick layer as favorably selected from known low dielectric constant materials and can thus suppress the parasitic factors generated in those wirings. The piezoelectric substrate 2 is covered at the upper surface with a sealing material 10A so that an oscillation space for the comb-shaped electrodes 3 is produced above.
An acoustic wave device similar to the conventional acoustic wave device 1 is disclosed in Patent Literature 1.
Recently, a demand for improving the reliability of mobile communications apparatuses has been intensified in view of the particular use under a high moisture condition for a long duration of time.
In the conventional acoustic wave device 1, the loading of a voltage under such a high moisture condition for a long duration of time may damage the resin material of the insulators 6, 8, and 10, and hence, result in declination of the reliability of the acoustic wave device 1.
An acoustic wave device similar to the conventional acoustic wave device 1 is disclosed in Patent Literature 1.